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- When we observe what we call fire

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we have this combustion reaction going on

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and then we see these flames

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what we're really observing

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are the three forms of thermal energy transfer.

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We're observing conduction, conduction,

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convection, convection,

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convection,

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and thermal radiation all at the same time.

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So I'll do this, thermal, thermal radiation,

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thermal radiation.

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And I could say thermal conduction,

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thermal convection, and thermal radiation,

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and the word thermal is just relating to things

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dealing with temperature.

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So what is the conduction going on?

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Well what we have going on in the fuel for our fire,

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so down here the fuel for our fire

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we have our classic combustion reaction

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and I encourage you to watch the video on that

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in our chemistry playlist, or chemistry section

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if you're interested in it.

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But you're taking carbon molecules and their bonds

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and then in the presence of oxygen and some heat

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you have a combustion reaction

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producing carbon dioxide and producing water

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and even more energy than you put into it,

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so it's producing a lot of energy.

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And that energy is going to excite

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the molecules that are around it,

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so you have these molecules,

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you have these molecules

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that end up getting a lot of kinetic energy.

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And remember, temperature is proportional

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to average kinetic energy.

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And so the ones that just combusted,

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or the molecules right near the reaction,

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or maybe the ones that were part of the reaction,

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they're gonna have this really high,

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they're gonna have this really high average kinetic energy

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'cause all of this energy was released

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from that combustion reaction.

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And then they're going to bump into other molecules

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that might not have quite as high kinetic energy,

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but then they're going to transfer that kinetic energy

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through these collisions.

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And we have a whole video on thermal conduction,

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but that's what's happening.

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Things with higher kinetic energy

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are bumping into things with lower kinetic energy

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and transferring some of that kinetic energy

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and they're transferring some of that momentum.

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So that is definitely happening

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when we look at a fire like this,

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that neighboring molecules are bumping

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into other molecules and transferring energy.

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Now convection, this is around the idea

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that hot air is less dense.

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So if we have a bunch of air particles here,

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so let's say this is the cool air up here,

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and it makes sense why cool air

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is going to be more dense than hot air.

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'Cause hot air, these particles have a much higher

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kinetic energy and so they're gonna bump

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into each other and they're gonna push each other

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much further away and get more seperation

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from each other because of that high average kinetic energy.

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But because the hot area is less dense,

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this area right over here is less dense, less dense,

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than the white area, these might be all the same molecule,

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I just made them in different colors to show

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the less dense area and the more dense area,

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it is going to rise, it is going to rise.

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Or you can even think of it

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as a more dense area.

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The more dense area is gonna fall around it,

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it's gonna fall around it, or try to go under it,

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because it is more dense,

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and then the less dense area is going to rise.

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And by doing that,

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you have the hotter molecules are moving upwards

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and then the cold molecules can go down

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to maybe the source of heat, the source of heat,

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the source of energy right over here

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to get heated up more.

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So this right over here is, so hot air rises,

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let me write this, the hot, hot air rises.

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And when we think about it

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it makes room for the cool air to come down

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and then get heated up again.

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And that's what's going on in the fire here.

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You see the combustion reaction,

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combustion reaction occurs right over there,

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let me just hit a different color,

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combustion reaction is occurring in the fuel,

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that super hot air, it's going to rise.

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And that's why it looks like these flames

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are kind of, they're moving upwards,

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they're kind of flickering upwards.

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And you'll also see, and actually we're gonna

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talk about this in thermal radiation,

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is that they also change color as they move upwards.

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But this whole idea of fire moving upwards

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and the hot air,

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if you put your hand up here it's gonna be much hotter

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than if you were to put your hand, say, right here,

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even though in theory this is closer to the flame

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than up here and that's because the hot air,

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the hot air is rising and it's making room for cold air,

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or cooler air, I guess I could say,

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to come down here and be part

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and get heated up again,

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and then it would rise up again.

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And so this convection,

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this idea of the hot air rising

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or the cold air falling,

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this is another form of heat transfer.

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Now the last form that we're observing

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when we're looking at fire is thermal radiation.

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And this is all around the idea

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that if you have charged particles being accelerated

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they're going to release electromagnetic radiation.

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And so you might say, Wait, wait, charged particles,

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where are those being accelerated here?

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Well, we have these molecules that are

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constantly being accelerated as they bump

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into each other,

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the one that's gonna transfer kinetic energy

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from, let's say this one bumps into that,

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it's going to accelerate that in different direction

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and even this one might go in another direction

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and acceleration is a change in velocity

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so it could be the magnitudinal velocity

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or your directional velocity.

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So as they're colliding you have all of these

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accelerational charged particles,

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so let me write this down,

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you have acceleration, acceleration

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of charged particles, charged particles,

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particles,

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and then that releases, that releases,

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electromagnetic radiation,

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that releases electromagnetic radiation.

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I know what you're thinking, Wait wait,

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hold on a second, Sal.

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Okay, I can buy that we have molecules

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that are being accelerated,

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but where is this charged,

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where are you getting this charged particles business?

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Well we have to remember these molecules

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are made up of atoms and atoms are made up of

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charged particles, are made up of protons and electrons.

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And so as you accelerate these

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and the more that you accelerate these

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the more radiation you are going to release.

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And you might say, Okay,

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you said I'm observing that in fire,

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where am I observing radiation?

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Well just the very fact that you can see the fire,

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the light emitted from the fire,

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that is electromagnetic radiation,

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that is electromagnetic radiation.

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It's just the electromagnetic radiation

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in the wavelengths that your eye

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considers to be visible light,

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or that your eye considers to be light.

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Even the particles up here,

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so even the particles up here

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that are still quite hot,

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they are also emitting electromagnetic radiation

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because they're all bumping into each other

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and their electrons and their protons

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are all getting accelerated in different ways,

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they're also releasing electromagnetic radiation,

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but it is at a slower wavelength

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than your eye is capable of perceiving as light.

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If you had an infrared camera

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you would see the flames being much larger.

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You would see them go all the way, all the way up here.

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And if you were to look at the flame closely

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you would see down here right where the

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combustion reaction is happening,

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the flame looks blue.

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And that's because the blue light

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is higher energy light,

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and that's because the particles are being accelerated

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more down here,

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and then it goes from blue to kind of a white

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to a yellow, to a red or to an orange, to a red,

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and then it, to your regular eyes, it disappears.

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But everything, everything that has some temperature

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is releasing electromagnetic radiation.

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And you're like, Okay, well that's all fine,

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I can see it but how is that a form of energy transfer?

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Well, if you're ever sat next to a flame

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you will feel the heat.

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In fact, even if the air between you and the flame is cold

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you would still feel like you're getting warm.

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So if this is a flame right over here,

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so that is fire, and let's say you have cold air,

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cold air, let's say you're at a campfire right over here,

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maybe it's 30, this air right over here is 30 degrees,

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if you are standing right over here

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you would still feel heat,

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you would still feel like you're getting warmed up.

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And that's because that electromagnetic radiation

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is being emitted from the air particles

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that we perceive as fire

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and then that can actually excite particles

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on your skin

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and it will transfer energy to your skin

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and so you feel like you are actually getting warmed up.

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I remember once, this is kind of a strange story,

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but I was on the highway

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and there was a car on fire

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and I was literally, we drove to the far lane

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because it was on fire,

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we're three lanes away from it

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and it kind of exploded.

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I don't think anyone was in it,

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hopefully no one was in it,

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but I remember right when it exploded

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it was an intense, immediate heat that we felt

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through the window of the car,

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and that was electromagnetic radiation.

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That was thermal radiation being released

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by these accelerated particles

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in the air around that explosion,

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which we perceived as a an explosion, or fire,

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but then it was warming up particles on my skin.